The invention relates to improvements in paper machine stock screens, and more particularly to an improvement in a foil construction which aids in movement of the stock through the screening mechanism.
Stock screens are used in the paper making process for aiding in cleaning the stock before it flows to the headbox to be dewatered to form a web. Such stock screens are conventionally tubular in shape with the stock being directed either to the inner surface or the outer surface of the tubular screen with the accepted stock flowing through the screen and the rejected stock including sheaves, particles, dirt and knots, not passing through the screen and flowing to the other end to be removed through a reject line. Typically, the stock to be screened is admitted at one end of the tubular screen, and the rejects flow out of the other end while the accepts are received by an annular chamber, with the annular chamber surrounding the screen in the case where the supply stock is delivered to the inside of the screen and the stock flows outwardly through the screen in the screening operation. Such devices are shown in U.S. Pat. No. 3,082,874, Justus, and U.S. Pat. No. 3,456,793, Salomon. In commercial operation, the screen will operate under pressure and stock will be pumped into one end of the tubular screen to enter the interior of the screen tangentially, and the pressurized screen will operate under a wide range of stock velocity. For aiding in the rapid flow of acceptable stock through the screen and preventing the fibers from building up on the screen surface, shaped foils are continuously moved around along the surface of the screen. Such foils have a rounded leading edge and a tapered trailing edge so that as they move along the screen, they create a pressure pulsation to cause a dispersal of the fibers and aid in the screening operation. These foils dispense the individual fibers for maximum screening efficiency employing a combination of centrifugal force, pressure differential and hydraulic action. The accepted fibers which flow through the screen are captured in an annular chamber and flow to the headbox or to a vat. The rejected fibers and lightweight impurities are directed toward a low pressure light rejects outlet at the lower end of the screen. To aid in the screening operation, it is desirable that the material being screened flow in an axial direction along the screen surface from the inlet end toward the discharge end. Attempts have been made to increase this flow and make it more uniform by constructing angular foils that extend in a generally helical direction such as shown in Martindale U.S. Pat. No. 2,835,173, U.S. Pat. No. Re. 24,677. However, because the foils have a critical shape, they are usually made by machining, or by a casting operation, and it is difficult and expensive to maintain the optimum shape and form the foils generally helical because of the difficulty of generating the helical shape of a foil along a cylindrical surface. Such machining is, therefore, prohibitive and the costs outweigh the advantages.
In accordance with the present invention, a structure has been provided wherein the foils can be manufactured by the standard less expensive method to extend axially, and yet they can be utilized to obtain a pumping action which causes the stock to flow axially along the screen surface. According to the invention, vane means are located on the surface of the screen, and extend in a helical direction so that they cause a flow of the stock from the delivery end of the screen toward the reject end. The vane preferably is in the form of a continuous rib on the surface of the vane facing away from the screen so that the rib in no way adversely affects the performance of the vane, and yet permits the vane to be made by less expensive machining methods without the necessity of machining the combined complex shape that is necessary to provide an airfoil configuration that extends helical.
Accordingly, an object of the present invention is to provide an improved stock screening mechanism for a paper making machine wherein the axial movement of the stock through a tubular screen is enhanced, and the screening function is improved.
A still further object of the invention is to provide a commercially practical pressure stock screen wherein the airfoil members which are moved along the surface of the screen can be of optimum design and be manufactured relatively reasonably, and yet the mechanism for moving the foils and the movement of the foils is utilized for moving the stock axially through the tubular screen.
Other objects and advantages and features of the invention, as well as equivalent structures which are intended to be covered herein, will become more apparent with the disclosure of the preferred embodiment in the specification, claims and drawings in which: